Synthesis of styrene-isoprene rubber

Abstract

This invention discloses a process for synthesizing random styrene-isoprene rubber comprising: (1) continuously charging isoprene, styrene, an initiator, and a solvent into a first polymerization zone, (2) allowing the isoprene and styrene to copolymerize in the first polymerization zone to total conversion of 60 to 95 percent to produce a polymer cement containing living styrene-isoprene chains, (3) continuously charging the polymer cement containing living styrene-isoprene chains and additional isoprene monomer into a second polymerization zone, wherein from 5 to 40 percent of the total amount of isoprene changed is charged into the second polymerization zone, (4) allowing the copolymerization to continue in the second polymerization zone to a conversion of the isoprene monomer of at least 90 percent wherein the total conversion of styrene and isoprene in the second polymerization zone is limited to a maximum of 98 percent, (5) withdrawing a polymer cement of random styrene-isoprene rubber having living chain ends from the second reaction zone, (6) killing the living chain ends on the random styrene-isoprene rubber, and (7) recovering the random styrene-isoprene rubber from the polymer cement, wherein the copolymerizations in the first polymerization zone and the second polymerization zone are carried out at a temperature which is within the range of 70° C. to 100° C., and wherein the amount of styrene charged into the first polymerization zone is at least 2 percent more than the total amount of styrene bound into the rubber.

What is claimed is:  
     
       1. A process of synthesizing random styrene-isoprene rubber having a low level of branching and a low vinyl content which comprises: (1) continuously charging isoprene, styrene, an organolithium initiator, and an organic solvent into a first polymerization zone, (2) allowing the isoprene and styrene to copolymerize in the first polymerization zone to total conversion which is within the range of about 60 percent to about 95 percent to produce a polymer cement containing living styrene-isoprene chains, (3) continuously charging the polymer cement containing living styrene-isoprene chains and additional isoprene monomer into a second polymerization zone, wherein from 5 percent to 40 percent of the total amount of isoprene is charged into the second polymerization zone, (4) allowing the copolymerization to continue in the second polymerization zone to a conversion of the isoprene monomer of at least 90 percent, wherein the total conversion of styrene and isoprene in the second polymerization zone is limited to a maximum of 99 percent, (5) withdrawing a polymer cement of random styrene-isoprene rubber having living chain ends from the second reaction zone, (6) killing the living chain ends on the random styrene-isoprene rubber, and (7) recovering the random styrene-isoprene rubber from the polymer cement, wherein the copolymerizations in the first polymerization zone and the second polymerization zone are carried out at a temperature which is within the range of about 70° C. to about 100° C., and wherein the amount of styrene charged into the first polymerization zone is at least 2 percent more than the total amount of styrene bound into the random styrene-isoprene rubber. 
     
     
       2. A cement of living styrene-isoprene rubber which is comprised of an organic solvent and polymer chains that are derived from isoprene and styrene, wherein the polymer chains are terminated with lithium end groups, wherein the polymer chains have a vinyl content of less than 10 percent, wherein less than 5 percent of the total quantity of repeat units derived from styrene in the polymer chains are in blocks containing five or more styrene repeat units, and wherein the molar amount of polar modifier in the cement of the living styrene-isoprene rubber is at a level of less than 20 percent of the number of moles of lithium end groups on the polymer chains of the living styrene-isoprene rubber. 
     
     
       3. A process as specified in claim  1  wherein the living chain ends on the random styrene-isoprene rubber are killed by the addition of a coupling agent. 
     
     
       4. A process as specified in claim  3  wherein the coupling agent is tin tetrachloride. 
     
     
       5. A process as specified in claim  1  wherein the amount of styrene charged into the first polymerization zone is at least 4 percent more than the total amount of styrene bound into the random styrene-isoprene rubber. 
     
     
       6. A process as specified in claim  5  wherein the copolymerization in the second polymerization zone results in a conversion of the isoprene monomer of at least 95 percent. 
     
     
       7. A process as specified in claim  6  wherein from 7 percent to 35 percent of the total amount of isoprene charged is charged into the second polymerization zone. 
     
     
       8. A process as specified in claim  7  wherein the total conversion reached in the first polymerization zone is within the range of about 75 percent to about 95 percent. 
     
     
       9. A process as specified in claim  8  wherein the copolymerizations in the first polymerization zone and the second polymerization zone are carried out at a temperature which is within the range of 75° C. to 95° C. 
     
     
       10. A process as specified in claim  9  wherein the amount of styrene charged into the first polymerization zone is at least 6 percent more than the total amount of styrene bound into the random styrene-isoprene rubber. 
     
     
       11. A process as specified in claim  10  wherein the copolymerization in the second polymerization zone results in a conversion of the isoprene monomer of at least 98 percent. 
     
     
       12. A process as specified in claim  11  wherein from 12 percent to 33 percent of the total amount of isoprene changed is charged into the second polymerization zone. 
     
     
       13. A process as specified in claim  12  wherein the copolymerizations in the first polymerization zone and the second polymerization zone are carried out at a temperature which is within the range of 80° C. to 90° C. 
     
     
       14. A process as specified in claim  13  wherein the organolithium initiator is an alkyl lithium compound. 
     
     
       15. A process as specified in claim  14  wherein the alkyl lithium compound is n-butyl lithium. 
     
     
       16. A process as specified in claim  1  wherein the polymerization in the first polymerization zone and the polymerization in the second polymerization zone are carried out in the absence of polar modifiers. 
     
     
       17. The cement of living styrene-isoprene rubber as specified in claim  2  wherein said cement of the living styrene-isoprene rubber is void of polar modifiers. 
     
     
       18. The cement of living styrene-isoprene rubber as specified in claim  2  wherein said polymer chains are derived from about 5 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 95 weight percent isoprene. 
     
     
       19. The cement of living styrene-isoprene rubber as specified in claim  2  wherein said polymer chains are derived from about 7 weight percent to about 40 weight percent styrene and from about 60 weight percent to about 93 weight percent isoprene. 
     
     
       20. A process of synthesizing random styrene-isoprene rubber having a low level of branching and a low vinyl content which comprises: (1) continuously charging isoprene, styrene, an organolithium initiator, a polar modifier, and an organic solvent into a first polymerization zone, (2) allowing the isoprene and styrene to copolymerize in the first polymerization zone to total conversion which is within the range of about 60 percent to about 95 percent to produce a polymer cement containing living styrene-isoprene chains, (3) continuously charging the polymer cement containing living styrene-isoprene chains and additional styrene monomer into a second polymerization zone, wherein from 5 percent to 40 percent of the total amount of styrene is charged into the second polymerization zone, (4) allowing the copolymerization to continue in the second polymerization zone to a conversion of the styrene monomer of at least 90 percent, wherein the total conversion of styrene and isoprene in the second polymerization zone is limited to a maximum of 99 percent, (5) withdrawing a polymer cement of random styrene-isoprene rubber having living chain ends from the second reaction zone, (6) killing the living chain ends on the random styrene-isoprene rubber, and (7) recovering the random styrene-isoprene rubber from the polymer cement, wherein the copolymerizations in the first polymerization zone and the second polymerization zone are carried out at a temperature which is within the range of about 70° C. to about 100° C., and wherein the amount of isoprene charged into the first polymerization zone is at least 2 percent more than the total amount of isoprene bound into the random styrene-isoprene rubber.